As an inorganic glass is excellent in its transparency and has excellent general properties such that optical anisotropy is low, it has been used in many fields as a transparent optical material. However, it has drawbacks in that it is easily broken due to its heavy weight and has bad productivity in processing optical parts. As a result, a transparent organic polymer material (optical resin) has been actively under development as a material for replacing the inorganic glass. Recently, high functionality and high quality in optical resins have been promoted. Optical parts obtained by molding and processing of these optical resins have further come into wide use in, for example, lens fields such as a spectacle lens for optical correction, a pickup lens in information recording devices such as CD, DVD and the like, a plastic lens for cameras such as a digital camera and the like, and sealing material fields in a light emitting element and the like.
One of the most important and fundamental properties as an optical resin is transparency. Up to now, as an optical resin having excellent transparency, there are known resins, for example, polymethyl methacrylate (PMMA), polycarbonate (BPA-PC), polystyrene (PS), methyl methacrylate-styrene copolymer (MS), styrene-acrylonitrile copolymer (SAN), poly(4-methyl-1-pentene) (TPX), polycycloolefin (COP), diethylene glycol bisallylcarbonate polymer (DAC), polythiourethane (PTU), and epoxy compounds such as 2,2-bis (4-hydroxyphenyl)propane (commonly called bisphenol A) type epoxy compound, an alicyclic epoxy compound and the like, and an epoxy resin which can be obtained using an acid anhydride compound as a curing agent.
Among these optical resins, polymethyl methacrylate (PMMA) is widely used as one of representative optical resins as it is superior in its transparency and has characteristics such that optical anisotropy is relatively low (low double refractive index), and molding and weather resistance are good. However, there are drawbacks in that its refractive index (nd) is low, i.e., 1.49 and the water absorption coefficient is high.
In the same manner, polycarbonate (BPA-PC), one of the representative optical resins, can be obtained by the polycondensation reaction of 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as bisphenol A what is commonly called) and a carbonate compound (for example, carbonyl chloride, diphenylcarbonate or the like), having characteristics such that transparency, heat resistance and impact resistance are excellent, and the refractive index (nd=1.59) is relatively high. As a result, it is widely used for optical purposes including an optical disk plate for information recording. However, it has drawbacks in that the chromatic aberration (dispersion of refractive index), double refractive index are relatively high and melting viscosity is high, thus making forming rather difficult. Needless to say, improvement of performance and characteristics is being made to overcome these drawbacks.
Diethylene glycol bisallylcarbonate polymer (DAC) is a thermosetting resin of a crosslinked high molecular structure to be obtained by casting radical polymerization of a monomer, i.e., diethylene glycol bisallylcarbonate. It has characteristics such that transparency and heat resistance are excellent, and the chromatic aberration is very low. Due to such characteristics, it is used the most for a general-purpose plastic spectacle lens for optical correction. However, there are drawbacks in that its refractive index is low (nd=1.50) and its impact resistance is rather inferior.
Polythiourethane (PTU) is a thermosetting resin of a crosslinked high molecular structure to be obtained by the reaction between a diisocyanate compound and a polythiol compound. It is an extremely excellent optical resin having characteristics such that its transparency and impact resistance are excellent, the refractive index is high (nd ≧1.6), and the chromatic aberration is relatively low. At present, it is used the most for the purpose of a high-quality plastic spectacle lens for optical correction in which the thickness is thin and its weight is light. However, there is room for further improvement only in the production process for lens requiring long time (1 to 3 days) for thermal polymerization molding.
In order to solve these problems and to produce optical parts such as optical lenses and the like with high productivity, there has been proposed a method to polymerize and mold a compound having radical polymerization ability (hereinafter referred to as polymerizable compound) for obtaining an aiming molded product within a short period of time in the presence of a compound (photopolymerization initiator) for initiating radical polymerization by irradiating light such as ultraviolet rays and the like (for example, JP92-180911A, JP88-207632, JP86-194401 or the like).
As a representative example of the polymerizable compound used for the photopolymerization, a (meth)acrylic acid ester compound is, for example, used. However, there have been proposed a (meth)acrylic acid ester compound or a (meth)acrylic (thio) ester compound or the like having a particular structure containing a bromine atom or a sulfur atom in order to obtain much higher refractive index and Abbe number. According to these methods, polymerization can be realized within a short period of time. However, considering a balance in the general properties such as transparency, dyeing property, optical properties (for example, refractive index, Abbe number or the like), thermal properties (for example, heat distortion temperature or the like), mechanical properties (for example, impact resistance, flexural strength, adhesion or the like) and the like, it is difficult to mention that the thus-obtained cured product (resin) is sufficiently satisfying as optical parts for spectacle lens materials or sealing materials for light emitting elements and the like.
As the polymerizable compound containing a sulfur atom, there has been proposed that a cured product of a bifunctional or multifunctional (meth)acrylic acid thioester compound to be derived from a thiol compound such as bis(2-mercapto ethyl) sulfide and the like is useful as a transparent resin having high refractive index (JP95-91262B, JP94-25232B and the like). JP94-2523B has described polymerization and curing by photopolymerization of the (meth)acrylic acid thioester compounds without presenting any concrete Examples. When optical parts are actually manufactured by photopolymerization using the (meth)acrylic acid thioester compound as described in the above patent publication, difference in composition of the polymerizable composition and photopolymerization conditions results in difference in physical properties of resin cured product to be obtained, so it is difficult to mention that they have physical properties for actual use in many cases.
Already known optical resins have excellent characteristics as described above, but, according to the present state, each of them has also drawbacks to overcome. Under these circumstances, it is required to develop optical resins such as spectacle lens materials or sealing materials for light emitting elements such that polymerization and forming can be performed by photopolymerization within a short period of time, in which transparency and optical properties (high refractive index and Abbe number) of the thus-obtained cured product or optical parts are excellent, and thermal properties and mechanical properties are good.